Home canning is not recommended for pumpkin butter or
any mashed or pureed pumpkin or winter squash. In 1989,
the USDA's Extension Service published the Complete
Guide to Home Canning that remains the basis of
Extension recommendations today, found in the September
1994 revision. The only directions for canning pumpkin
and winter squash are for cubed pulp. In fact, the
directions for preparing the product include the
statement, "Caution: Do not mash or puree."

In accordance with the USDA recommendations, the
University of Georgia Cooperative Extension Service does
not have a recommendation for canning these products
either. There are not sufficient data available to allow
establishing safe processing times for any of these
types of products. It is true that previous USDA
recommendations had directions for canning mashed winter
squash, but USDA withdrew those recommendations and any
publications preceding the Complete Guide to Home
Canning (September 1994) are considered out of date.

Some of the factors that are critical to the safety
of canned pumpkin products are the viscosity
(thickness), the acidity and the water activity. Studies
conducted at the University of Minnesota in the 1970's
indicated that there was too much variation in viscosity
among different batches of prepared pumpkin purees to
permit calculation of a single processing recommendation
that would cover the potential variation among products
(Zottola et. al, 1978). Pumpkin and winter squash are
also low-acid foods (pH > 4.6) capable of supporting the
growth of Clostridium botulinum bacteria which can cause
the very serious illness, botulism, under the right
storage conditions. If the bacteria are present and
survive processing, and the product has a high enough
water activity, they can thrive and produce toxin in the
product.

More recent research with pumpkin butter has been
done at the University of Missouri. Pumpkin butter is
mashed or pureed pumpkin that has had large quantities
of sugar added to it, but not always enough to inhibit
pathogens. Sometimes an ingredient such as vinegar or
lemon juice is added to the formulation to increase the
the acidity (decrease the pH). However, pumpkin butters
produced by home canners and small commercial processors
in Missouri have had pH values as high as 5.4. In fact,
the pH values seemed to be extremely variable between
batches made by the same formulation (Holt, 1995).

It is not possible at this point to evaluate a
recipe for pumpkin or mashed squash for canning
potential by looking at it. At this point, research
seems to indicate variability of the products is great,
and in several ways that raise safety concerns. It is
best to freeze pumpkin butters or mashed squash.